vfs_set_lowermount
error = vfs_set_lowermount(mp, um->um_uppervp->v_mount);
vfs_set_lowermount(mp, NULL);
vfs_set_lowermount(mp, NULL);
error = vfs_set_lowermount(mp, lowerrootvp->v_mount);
error = vfs_set_lowermount(mp, lowerrootvp->v_mount);
error = vfs_set_lowermount(mp, lowerrootvp->v_mount);
int vfs_set_lowermount(struct mount *, struct mount *);